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Clay minerals in the Upper Permian evaporite deposits from the Mali Kukor, Vranjkovići, and Slane Stine (Dalmatia, Croatia)

The studied evaporite deposits (Mali Kukor, Vranjkovići, and Slane Stine) are located in the central part of middle Dalmatia, Croatia. In this region, Upper Permian evaporite sediments were deposited under favourable conditions onto the Variscan basement around the northern margins of Gondwana. The characteristics of these sediments are indicative of deposition in supratidal and sabkha settings (i.e., early diagenetic dolomites and evaporites) within a shallow epicontinental marine environment with highly varied coastlines, bays, and lagoons (DEDIĆ et al., 2018). Mineralogical composition of Upper Permian evaporite sediments in central part of middle Dalmatia was investigated to determine the type and extent of outcrop weathering alteration and to explore the preservation of diagenetic clays in changing geochemical environments. The clay minerals of Upper Permian evaporite sediments have been studied by X-ray diffraction (XRD) on 21 oriented samples. The evaporite sediments were collected from the outcrops in deposits, while clastic sediments from the surrounding area (Mali Kukor, Garjak and Suhač). The deposit of Mali Kukor is dominated by gypsum, anhydrite and calcite, Slane Stine by gypsum, anhydrite and dolomite, while Vranjkovići by gypsum and dolomite. The clay mineral association of insoluble residue of evaporite sediments in these deposits mainly consists of illite, illitesmectite, smectite, chlorite and kaolinite. Clay minerals in Mali Kukor vary from illite, illite-smectite and smectite in upper evaporite units to illite, chlorite and kaolinite in lower units. In Slane Stine evaporite deposits, the clay minerals are represented by illite, chlorite and kaolinite, while in Vranjkovići deposits the dominant mineral in the clay fraction is illite, with occasionally appearances of chlorite and kaolinite. Clay minerals in clastic sediments are represented by chlorite, illite and kaolinite, while in one locality vermiculite and illite are present. A variety of clay minerals are commonly found in shallow epicontinental marine environment and clay minerals also form a considerable portion of the sedimentary successions accumulated under evaporitic conditions in continental settings of the past. The clay minerals are products of weathering of rocks or precipitation reactions in saline solutions where surface conditions favour hydrous minerals (MEUNIER, 2005). The type of the clay minerals forming from rock weathering is a function of precursor rock type, chemistry of interacting fluids, and intensity of weathering i.e., humid vs. arid climates ; where moderate weathering produces smectite and intense weathering produces kaolinite (MEUNIER, 2005). The produced clay minerals behave as detrital minerals so they can be carried to sedimentary basins via erosional processes. Mechanisms of clay formation include: transformation from detrital precursors via solid-state substitutions or micro-scale dissolution/precipitation reactions, topotactic growth onto precursor minerals, and direct precipitation from solution or neoformation (HAY & KYSER, 2001 ; DEOCAMPO et al., 2009 ; BRISTOW & MILLIKEN, 2011). It is concluded that the clay minerals in Upper Permian evaporite sediments are detrital in origin, they reflect the character of their source material, and are slightly modified in their depositional environments. The illite, chlorite and kaolinite assemblage is typical for clastic sedimentary environments with a medium to high dynamics, while smectite indicates high hydrodynamics (CHAMLEY, 1989). Diagenesis and transformation can be determined in Vranjkovići deposits where illite is the dominant clay mineral, while upper units in Mali Kukor deposit is predominated by smectite and mixed- layer illite-smectite which point to different depositional conditions.

evaporite deposits, Upper Permian, clay minerals

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